1. Field of the Invention
The present invention relates to a projection type image display apparatus using an optical modulation image display device as represented by a liquid crystal display device as a light valve.
2. Description of Prior Art
Heretofore, there has been well known, in order to realize a large size image display, a method of forming an optical image according to a video signal on a relatively small light valve as a variation of the optical characteristic, irradiating this optical image by illuminating light, and projecting it on a screen by a projection lens. In the projection type image display apparatus of this kind, the resolution of the projected image is mainly determined by the resolution of the light valve, and since the optical output is enlarged by intensification of the light source, a projection type image display apparatus having high resolution and large optical output can be realized by the use of a light valve having a high resolution, even if its display area is small. Recently, a method of using a liquid crystal display device as a light valve has been noted with attention. For example, Morozumi et al. proposes in "LCD Full-Color Video Projector", SID 86 Digest, p. 375, a method of obtaining a full color projection image by using three liquid crystal display devices.
Construction of the conventional projection type image display apparatus using a liquid crystal display device as a light valve as such is explained.
Light irradiated from a lamp is converted into nearly parallel rays by a condenser optical display unit, and is then passed through a the liquid crystal display device, and enters a projection lens. In the liquid crystal display device, an optical image according to the video signal is formed, and this optical image is projected on the screen by the projection lens. The audio signal is processed by an audio signal processing circuit to drive the speaker. As a lamp, a halogen lamp, a xenon lamp, a metal halide lamp, etc. may be used. Starting and stopping of the ballast and igniter circuit for lighting the lamp are performed simultaneously with switching ON or OFF of the main power switch.
However, in case of using a metal halide lamp in such a conventional projection type image display apparatus, in order to re-ignite the metal halide lamp, a waiting time is required until the internal pressure of the lamp is lowered, i.e., until the temperature of the lamp is lowered. As such, due to the long duration before re-igniting, the metal halide lamp has the following problems. For instance, when a viewer turns a main power switch off by erroneous operation while using a remote controller, the lamp is instantly put off, but even if the viewer, becoming aware of his erroneous operation, puts the main power switch on, normally the lamp is not instantly lighted. This state is extremely inconvenient to the viewer.
If a high voltage pulse of about 30 KV is used only for re-starting, the metal halide lamp can be instantaneously ignited, but it involves such a problem that the electrode constituting the lamp is significantly deteriorated at the time of application of a high voltage pulse.
Furthermore, in starting the lamp, the metal halide lamp is under application of a high voltage pulse of several KV from the ballast and igniter circuit for several seconds, so that, in starting the lamp, i.e., during the time of application of a high voltage pulse to the lamp, noise comes into the audio signal due to the interference from the ballast and igniter circuit to generate an unpleasant sound.